Most large earthmoving vehicles and the like today employ an air-over-oil actuated brake system in order to reduce the amount of operator effort required to apply the wheel brakes of such vehicles. The air assist is commonly accomplished by attaching a separate air actuation chamber at one end of a hydraulic brake master cylinder. In such arrangements, a push rod is used to interconnect the pressure plate of the air chamber with the power piston of the master cylinder so that the energy of the pressurized air is transmitted through the movement of the pressure plate to the power piston by the push rod in order to actuate the master cylinder. However, most of such air chambers utilize a rolling rubber diaphragm for sealing purposes. Such diaphragms are subject to rupture, making the brakes inoperative. Also, such air chambers and their associated master cylinders are extremely complex and contain a relatively large number of components which greatly adds to the overall cost of the brake system.
The relatively small amount of oil necessary to actuate the brakes of smaller vehicles permits the use of a correspondingly small reservoir for supplying makeup oil to the brake system during operation. Such small reservoirs are aptly contained directly in the master cylinder housings of such brake systems without making such housings unduly large and complex. However, in large earthmoving vehicles which frequently utilize large capacity oil cooled, multiple disc-type brakes, a relatively large quantity of oil is necessary to actuate such brakes. Consequently, this requires a correspondingly large reservoir for the makeup oil. Due to space limitations and various other reasons, such large reservoirs and their master cylinders are usually mounted at different locations on the vehicle. Because of the distance the oil must travel between the reservoir and the master cylinder, the response time for replenishing the master cylinder is relatively slow. In an emergency situation, this may result in a delay of the desired application of the brakes and contribute to an already hazardous situation.